NOTES AND CORRESPONDENCE. Trends in the North Atlantic Oscillation Northern Hemisphere Annular Mode during the Twentieth Century*

336 JOURNAL OF CLIMATE VOLUME 16 NOTES AND CORRESPONDENCE Trends in the North Atlantic Oscillation Northern Hemisphere Annular Mode during the Twentieth Century* GREGORY M. OSTERMEIER AND JOHN M. WALLACE Department of Atmospheric Sciences, University of Washington, Seattle, Washington 28 December 2001 and 9 August 2002 ABSTRACT The North Atlantic Oscillation Northern Hemisphere annular mode (NAO NAM) has exhibited a trend over recent decades toward high index values. It has been argued that this trend is unprecedented in the historical record and that it may be attributable to anthropogenic forcing. This study compares and contrasts wintertime trends during the recent period of rising NAO NAM indices with trends earlier in the century when the indices declined. This analysis finds that the spatial patterns of the trends in sea level pressure (SLP) and surface air temperature (SAT) are markedly different in the two periods. The prior SLP trends were large only in the Atlantic sector, whereas the recent trends of the last few decades were more hemispheric in extent. As a result, trends in a more localized NAO index are roughly comparable for those two intervals, but the recent upward trend in the hemispheric NAM index is more prominent than the earlier downward trend. 1. Introduction Surprisingly large trends in the wintertime climate of the extratropical Northern Hemisphere (NH) over the past 30 years have been noted in a number of recent observational studies. Sea level pressure (SLP) over the Arctic has decreased (Walsh et al. 1996); the stratospheric polar vortex has become colder and has been persisting longer into spring (Waugh et al. 1999); surface air temperature (SAT) over much of Eurasia and North America has warmed substantially (Jones 1994; Hurrell 1995; Nicholls et al. 1996); regional precipitation patterns have shifted (Hurrell and van Loon 1997); and water mass characteristics of the Arctic Ocean and Nordic Seas have changed (e.g., Grotefendt et al. 1998; Blindheim et al. 2000). These and other changes over the past three decades have been linked to a shift in the dominant mode of variability of the SLP over and around the North Atlantic Ocean, widely regarded as a regional teleconnection pattern known as the North Atlantic Oscillation (NAO; Walker 1924; van * Joint Institute for the Study of the Atmosphere and Ocean Contribution Number 898. Corresponding author address: Gregory M. Ostermeier, Joint Institute for the Study of Atmosphere and Ocean, University of Washington, Box 354235, Seattle, WA 98195-4235. E-mail: greg@atmos.washington.edu Loon and Rogers 1978). Thompson and Wallace (1998, 2000) and Wallace (2000) have argued that the NAO is not a regional North Atlantic phenomenon in its own right but rather the local signature of a broader, hemispheric-scale NH annular mode (NAM). The high values of the NAO index of the past two decades and the trend toward higher values of the NAM index over the past 30 years or so have been called unprecedented in data records extending back at least a century (Hurrell 1995; Thompson et al. 2000). One mechanism that could account for long-period North Atlantic variability is an NAO- or NAM-like atmospheric pattern either stochastically forcing or interacting with the ocean locally, or perhaps remotely with the Tropics, as suggested by Hoerling et al. (2001). Another possibility is that the recent NAO NAM trend might be a reflection of changes in the stratospheric circulation occurring in response to increasing greenhouse gas concentrations (e.g., Shindell et al. 1999) or ozone depletion (Kindem and Christiansen 2001). This study examines NAO NAM-related variability spanning the last century. Trends in different periods of the record are compared and contrasted with a view toward assessing the uniqueness of the recent trend in the NAO NAM as well as the subtle distinctions between the temporal variations in the NAO and the NAM. The datasets used in this study are described in section 2, results are presented in section 3, and a brief discussion of the results follows in section 4. 2003 American Meteorological Society

15 JANUARY 2003 NOTES AND CORRESPONDENCE 337 2. Data sources and analysis procedures The primary gridded datasets utilized in this study are SLP from the National Center for Atmospheric Research (NCAR) data library (Trenberth and Paolino 1980) and SAT from the Climate Research Unit of the University of East Anglia (Jones 1994; Parker et al. 1995; Jones et al. 2001). These data are formatted on 5 latitude 5 longitude grids, and the SLP data are limited to the area poleward of 20 N. Also used are SLP and SAT data poleward of 50 N from three catalogs of station data: the Environmental Working Group Arctic Climatology Project Arctic Meteorology and Climate Atlas version 1.0 (Fetterer and Radionov 2000), the North Atlantic Climatological Dataset version 1 (Dahlström et al. 1995; Frich et al. 1996), and the World Monthly Surface Station Climatology (Spangler and Jenne 1990). Collectively these three datasets (hereafter referred to as station data ) contain SLP and SAT records for over 300 high-latitude locations for at least some part of the century. Because few of these station records have been updated through the 1990s, an augmented version of the station data was used, which incorporated SLP and SAT anomalies from the (National Centers for Environmental Prediction) NCEP NCAR reanalysis (Kalnay et al. 1996) as a proxy for missing station data. Use of the reanalysis data as a proxy for the station data is justified by the fact that the two were found to be generally very highly correlated, even after the trends were removed. The amplitude of the added reanalysis data was determined by a least squares best fit based on the common period of record. The fitting procedure was necessary to adjust for the potentially widely differing variances, particularly for SAT, between coastal stations and reanalysis data from the nearest grid points. A station series was supplemented to fill in missing data only for the years 1979 2000, and only if the station had data for at least half the months in the interval 1958 2000. The NAO index employed in this study is the standardized difference between the standardized wintermean SLP anomalies at Lisbon, Portugal, and Stykkisholmur/Reykjavik, Iceland, following Hurrell (1995). The NAM index used here is the standardized leading principal component of the gridded 1900 2000 monthly mean area-weighted NH SLP, following Thompson and Wallace (1998). The winter season is defined as extending over December January February March (DJFM), and individual winter-mean values are calculated only if data for all four months are available. Trends are estimated in terms of linear least squares best fits to the winter-mean data for the interval under consideration. They are expressed not as changes per decade, but as incremental changes SLP or SAT over the specific interval for which the trend is computed. Trends are calculated only for grid points and stations at which at least 75% of the winter-mean values for the interval are available. FIG. 1. Linear trends in units of std devs of the NAO index (lower right) and the NAM index (upper left), smoothed with a five-term Gaussian filter, for all possible intervals longer than 10 yr for the 1900 2000 period of record. Greater distances from the diagonal line bisecting the figure correspond to longer trend intervals. Asterisks mark the 1920 69, 1949 69, and 1969 2000 intervals examined in detail and shown in Fig. 2. The filtered indices are plotted along the axes. The NAO index half of the figure is best viewed with the page turned to the right one-quarter rotation. 3. Results Figure 1 provides an overview of the trends in the NAO and NAM indices over the twentieth century as manifested in time series that have been lightly smoothed with one pass of a 5-yr Gaussian filter, which are plotted along the figure axes. Linear trends, fitted to all possible choices of interval start and end dates, are plotted in the interior with units of standard deviations of the smoothed time series over the interval in question. Trends for intervals shorter than 10 years, which lie along the diagonal, are not shown. Downward trends (bluish shading) are prevalent prior to the late 1960s and stronger upward trends (reddish shading) are prevalent from that time onward. Trend intervals selected for intensive analysis (in particular, 1920 69 and 1969 2000) are indicated by asterisks. They correspond to maxima in the plot but are broadly representative of the contrasting trends in the earlier and later parts of the century. Mindful of the possibility that the upward trend in the indices toward the end of the century might be an anthropogenically induced secular trend, the latter interval is defined as extending until the end of the available record rather than ending in the early or mid- 1990s, which would have yielded slightly larger trend values. Unsmoothed time series of the NAO and NAM indices are shown in Fig. 2, together with the intervals selected for detailed investigation of the trends. Nu-

338 JOURNAL OF CLIMATE VOLUME 16 TABLE 1. NAO index NAM index Linear trends in units of std dev over the full interval in the NAO and NAM indices. 1920 69 1969 2000 1.59 0.94 1.35 1.92 FIG. 2. Standardized DJFM unsmoothed seasonally averaged indices of the NAO and NAM. Linear trends for 1920 69 and 1969 2000 are indicated by thick black lines, and linear trends for 1949 69 are indicated by thick gray lines. merical values of the trends during these intervals are shown in Table 1. In both Figs. 1 and 2 and in Table 1 the trends in the two indices are fairly similar, but the NAO index exhibits a slightly more balanced V shape, whereas in the NAM index the upward trend in the later part of the century eclipses the downward trend in preceding decades. Next, the spatial patterns associated with these trends are considered. For reference, maps of SLP and SAT regression patterns for the NAO and NAM time series are shown for the gridded data in Fig. 3. The linear trends in the DJFM mean gridded SLP and SAT fields for the periods 1920 69, 1949 69, and 1969 2000 are shown in Fig. 4. The corresponding regression and trend maps for the station data are shown in Figs. 5 and 6. SLP trends for 1920 69 were dominated by a North Atlantic dipole pattern with rising SLP over the area around southern Greenland and Iceland and falling SLP throughout the lower-latitude Atlantic. The poles coincide with the locations of maximum amplitude in the regression patterns of Fig. 3, but the strongest trends were more concentrated within the Atlantic sector. Warming occurred over Labrador and much of the Middle East stretching into central Eurasia, and cooling occurred over northwestern Russia and Scandinavia as well as the southeastern United States. Most of the stations with data adequate for estimating 1920 69 trends are located around the North Atlantic. Trends in the station data over this interval (Figs. 6a,b) are consistent with those based on the gridded data. Mindful of the limited data coverage during the first half of the century, trends were also investigated during the shorter 1949 69 interval when the declines in the index values were particularly pronounced, as shown in Figs. 1 and 2. The trend patterns for this interval are generally similar to those for the full 1920 69 interval. A North Atlantic dipole is again apparent in SLP, and a North Atlantic quadrupole in SAT is observed. Over the North Atlantic sector the station data closely match the gridded data. They clearly show the northern portion of the SLP dipole and the opposing SAT trends in western Greenland and all of northern Europe. A feature more evident in the station data is the band of weak but consistently negative SLP trends stretching along the Arctic coast of Russia. The strong cooling over the region of the Barents and Kara Seas is consistent with the trend toward a more northerly surface geostrophic wind component as implied by the trends toward higher pressure to the west and lower pressure to the east of this region. The gridded data for the recent period 1969 2000 indicate that SLP fell across a broad area covering the Arctic Basin and Siberia and rose in a belt extending from the Canadian Atlantic coast to the eastern end of the Mediterranean. This pattern looks quite like the SLP regression patterns in Fig. 3, except over the Pacific, and its hemispheric scale is in stark contrast to the SLP trends for the earlier intervals that project onto the regression patterns primarily over the North Atlantic and lower latitudes. SAT trends for 1969 2000 are more FIG. 3. DJFM monthly (a), (c) SLP and (b), (d) SAT anomalies for 1950 2000 regressed onto (a), (b) NAO and (c), (d) NAM indices. Contour intervals are indicated in the scale below each column.

15 JANUARY 2003 NOTES AND CORRESPONDENCE 339 FIG. 5. (a), (c) DJFM monthly station SLP and (b), (d) station SAT anomalies for 1950 2000 regressed onto (a), (b) NAO and (c), (d) NAM indices. Positive values are black, and negative values are gray. Magnitudes are indicated in the scale to the lower left of each column. Only those stations with data for at least one-third of the winters of the century are plotted. FIG. 4. Linear DJFM seasonally averaged trends in (a), (c), (e) gridded SLP and (b), (d), (f) SAT. (a), (b) Trends for 1920 69, in units of mb (50 yr) 1 and K (50 yr) 1 ; (c), (d) 1949 69, in mb (21 yr) 1 and K (21 yr) 1 ; (e), (f) 1969 2000, in mb (32 yr) 1 and K (32 yr) 1. Contour intervals are indicated in the scale below each column. Because of missing and poor quality SLP data from early in the century at very high latitudes, that area is not plotted in (a). difficult to interpret in terms of regional patterns because they were dominated by warming that was strongest over the high-latitude continents, quite possibly a response to increasing greenhouse gas concentrations (Mitchell et al. 2001). Weak cooling occurred over Labrador and western Greenland and parts of the Middle East. The magnitude of the cooling is sufficient to be visible in Fig. 4f only at a few grid points in those areas, but the absence of warming is notable. The station SLP trends are consistent with those in the gridded data, with moderate decreases throughout the Arctic Basin and the North Atlantic. SAT trends were positive virtually everywhere in the high-latitude station data, except over northeastern North America. The strong resemblance between these recent SAT trends and the regression patterns in Figs. 3 and 5 is consistent with the attribution of much of the spatial structure in the trends to the NAO NAM (Graf et al. 1995; Hurrell 1995; Thompson et al. 2000). The more localized spatial distribution of the SLP trends in the earlier part of the century projects more strongly onto the regression pattern for the NAO index than onto that for the NAM index, whereas the broader distribution of the SLP trends observed over the end of the century projects more strongly onto the NAM index than the NAO index. These distinctions are evident in Table 1, which shows the trends in the NAO and NAM indices for the two intervals. The distinction between the patterns of the trends in these intervals is also evident in the pattern correlations shown in Table 2. The trends during the two intervals exhibit similar correlations with the NAO, whereas the latter interval exhibits much stronger correlation with the NAM than does the earlier interval. 4. Discussion Since the intervals over which the trends were computed were not chosen a priori, the statistical significance of the trends cannot be assessed by means of formal tests. As an alternative, a Monte Carlo approach was employed to assess the probability of finding trends in 101-yr, standardized, first-order autoregressive time series at least as large as the observed trends. The random time series have the same 1-yr lag correlations as

340 JOURNAL OF CLIMATE VOLUME 16 TABLE 2. Pattern correlations for the gridded NAO and NAM regression patterns (Figs. 3a,c) and the gridded SLP trend maps for the two primary intervals (Figs. 4a,e). For each calculation both patterns were weighted by the square root of latitude to give equal areas equal weighting. NAO NAM 1920 69 1969 2000 0.63 0.55 0.72 0.78 FIG. 6. Linear DJFM seasonally averaged trends in station (a), (c), (e) SLP and (b), (d), (f) SAT. (a), (b) Trends for 1920 69, in units of mb (50 yr) 1 and K (50 yr) 1 ; (c), (d) 1949 69, in mb (21 yr) 1 and K (21 yr) 1 ; (e), (f) 1969 2000, in mb (32 yr) 1 and K (32 yr) 1. Positive values are black, and negative values are gray. Magnitudes are indicated in the scale to the lower left of each column. the observed series for the 1900 2000 period (0.31 for the NAO index and 0.21 for the NAM index). As in the analysis of the observed indices, only trend intervals of 50- and 32-yr lengths were examined, but no constraints were set on the sign of the trends or on their location within the random series. Based on 10 000 random time series the p values for the 1920 69 and 1969 2000 trends in the NAO index and the NAM index are shown in the top rows of Table 3. These p values estimating the likelihood of the trends may be affected by the observed autocorrelations in the indices being, at least in part, a reflection of the trends whose statistical significance are being assessed. If the time series of the indices are adjusted to eliminate the observed trends during the two periods of interest, the 1-yr lag correlations drop to 0.15 for the NAO index and 0.09 for the NAM index, and the corresponding p values are as indicated in the bottom rows in Table 3. With this adjustment, the downward trend in the NAO index in 1920 69 and the upward trend in the NAM index in 1969 2000 appear to be marginally significant; the other trends remain not at all outstanding. The conclusions regarding the trends are not conditioned by our particular choice of indices. For example, using the leading principal component of North Atlantic sector SLP as the index for the NAO yields similar results. We have also verified that the spatial patterns depicted in Figs. 4 and 6 are robust with respect to small changes in the endpoints of the trend intervals or to changes in analysis procedures (e.g., estimating the trend as the difference between multiyear averages of data from ends of the intervals). The more regionally concentrated trend pattern observed during 1920 69 is suggestive of atmosphere ocean interaction localized in the North Atlantic sector, whereas the broader, more hemispheric pattern observed during the recent period 1969 2000 is suggestive of a connection with the even larger trends in the stratospheric polar vortex observed during this period or possibly an annular mode response to tropical forcing. However, in view of the limited data coverage during the first half of the twentieth century, the marginal statistical significance of the observed trends, and the indications of a weak downturn in the NAM index since the mid-1990s, we remain open to other interpretations. Acknowledgments. This work was supported by Grant ATM 9805886 from the National Science Foundation. This publication was also partially funded by the Joint Institute for the study of the Atmosphere and Ocean under NOAA Cooperative Agreement NA67RJ0155. TABLE 3. Probability of trends of the same length and the same or greater magnitude as those observed in the NAO index and the NAM index arising in random time series, generated as first-order autoregressive processes with the same length, variance, and lag-1 autocorrelation as the observed indices. The calculations were repeated generating random time series having the lag-1 autorcorrelation of the NAO and NAM indices adjusted to remove the 1920 69 and 1969 2000 trends. NAO index (a 1 0.31) NAM index (a 1 0.21) Adjusted NAO index (a 1 0.15) Adjusted NAM index (a 1 0.09) 1920 69 1969 2000 0.16 0.74 0.05 0.60 0.78 0.15 0.57 0.06

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